How to make a transparent reflective surface with transparent film
In the end, I went with compositing an opaque background image underneath the transparent layer and that worked alright, but before that I used this node setup to set the alpha value based on the value of each pixel.



The Subtract node is used as a minimum threshhold. You might have to tweak that depending on your setup.

The first Multiply is just a messy way to get a definite 1 or 0 on whether or not the pixel should be visible or not.

The second Multiply is used to set the final alpha value for the partially transparent pixels.

The Add node at the end is just to make sure that no previously-opaque pixels become transparent.

In my case it ended up looking like this: